Thermal insulation



Jan. 3, 1961 c. MATSCH ET AL THERMAL INSULATION Filed April 26, 1956INVENTORS LADISLAS C. MATSCH ARTHUR W. FRANCIS VM Q. m

THERMAL INSULATION Ladislas C. Matsch, Kenmore, and Arthur W. Francis,

Dobhs Ferry, N.Y., assignors to Union Carbide Corporation, a corporationof New York Filed Apr. 26, 1956, Ser. No. 580,897

12 Claims. c1. 252-62) This invention relates to an improved insulatingmaterial having a high resistance to all modes of heat transfer, andparticularly concerns a low temperature insulating material for use invacuum jackets of containers.

In the conservation and conveying of low-temperature commercialproducts, for example, perishable commodities which must be held at lowtemperatures for substantial periods of time, and of volatile materialssuch as liquefied gases having boiling points at atmospheric pressurebelow 233 K., for example, liquid oxygen or nitrogen, a major problemencountered is the control of heat leak to the material which, in thecase of liquefied gases, results in loss due to evaporation. In theconventional double-walled liquid oxygen containers, the space betweenthe walls is suitably insulated to limit this evaporation loss. Up tonow, straight vacuum-polished metal or poWder-in-vacuum insulation ofthe type disclosed in U.S. Patent No. 2,396,459, has been used toinsulate the space between the walls. However, a general disadvantage ofstraight vacuum-polished metal insulation is the necessity ofmaintaining an extremely high vacuum. Powder-in-vacuum heat insulationis less sensitive to the presence of small traces of air in theinsulation space, however, significant quantities of heat from the atmosphere are, nevertheless, transmitted from the external shell of thecontainer to the inner vessel.

There is a great commercial need for efiicient insulating materialscapable of meeting more rigid and exacting requirements, and which willprovide even lower thermal transmission than those afforded by either ofthe above described insulating systems. Provisions of such ma terialswould permit study and development of important new and improved controltechniques for many processes and products.

The present invention is based on the discovery of important behaviorcharacteristics of finely divided insulating powders used as heatinsulation. The geometry of the insulating material has the greatesteffect on solid heat conduction, the rate of heat transfer by conductionvarying directly with the cross sectional area and inversely with thelength of the heat path. The contacts between powder particles areusually of relatively small cross sectional area. Consequently, inpowders, the area available for conductive heat transfer is aninfinitesimal fraction of the insulated area. It would seem, therefore,that by reducing the powder size, the resistance to the flow of heat byconduction and the permissible temperature gra dient would becorrespondingly increased. However, our

come the predominant mode of heat transmission through very fine powderseven at low temperature. 1

In order to achieve acceptable combinations of the 2,967,152 PatentedJan. 3, 1961 various modes of heat transfer in an insulating material,means must be provided for reducing the infra-red radiation transparencyeffects accompanying small particle sizes without incurring anappreciable increase in heat transfer by solid conduction.

It is, therefore, an object of the present invention to provide in aninsulation system, improved means for reducing the undesirable radiativeeffects in low conductive powders of relatively small particle sizes.

Another object of the present invention is to provide in a vacuum-solidinsulation system, an improved and efficient insulating material havinga relatively high resistance to the passage of heat by conduction and byradiation.

Still another object of the present invention is to provide in avacuum-powder insulation system, additive material for minimizing thepassage of heat by radiation without increasing the passage of heat byconduction to any significant degree.

Yet another object of the present invention is to provide a novelimprovement in insulation material for use in an insulation system whereradiation would otherwise be the predominant mode of heat transfer.

Other objects, features and advantages of the present invention will beapparent from the following detailed description.

In the drawings:

Fig. l is a view of a double-walled container forliquefied gas embodyingthe principles of the invention; and

Fig. 2 is a view of an enlarged section taken along line 2-2 in Fig. 1,showing the insulating material of the present invention.

According to the present invention, the undesirable radiative effectsaccompanying the use of .a finely divided low heat conductive powder ina vacuum insulating space may be substantially reduced and minimized byincorporating therein a multiplicity of radiant heat barriers capable ofinterrupting the passage of infra-red radiation rays withoutsignificantly increasing the thermal conductivity across the insulatingspace. This may be accomplished by providing throughout the powderinsulation a series of spaced, randomly dispersed radiation opaque orreflective bodies, which in combination with each other constitute aplurality of discontinous radiation barriers.

it is to be understood that the term radiant heat barrier as used hereinis intended to apply to radiation opaque or radiant heat energyimpervious materials or materials having lightly reflective surfaces,and capable of hindering, interrupting or stopping the penetration ofinfra-red heat rays through the insulation space either by radiant heatreflection, radiant heat absorption or both.

The term vacuum as used hereinafter is intended to apply tosub-atmospheric pressure conditions not substantially greater than 5000microns of mercury, and preferably below 500 microns of mercuryabsolute.

In applying the principles of the invention to a conventionalpowder-vacuum insulation system, for example, a powder-vacuum typeinsulation system of the type dis closed in U.S. Patent No. 2,396,459,the combination of finely divided low conductive insulating powder withrelatively small bodies of material having heat reflective or heatabsorbing properties provides effective insurance against passage ofheat leak.

A practical illustration of an apparatus embodying the invention shownin Fig. 1 may comprise a double-walled insulating vessel having spacedparallel walls 10 defining an evacuable insulating space 11 therebetweenfor the reception of a solid, powder-type insulation mixture 12embodying the principles of the invention. The insulation mixture 12 maycomprise a finely divided agglomerate of low heat conductive material 13in which particles or bodies 14 of radiant heat barriers areintermingled.

The low heat conductive powder 13 used in the insulation of theinvention should be a material which may be produced in fine particlesizes, or can be readily reduced to a fine powder. It should be strongand rigid enough to fill the insulation space, and not pack downexcessively during normal service. Among the insulating powders whichgive excellent results are finely divided silica silicates such asperlite, alumina, magnesia and other similar metallic oxides, and carbonblack, finely divided silica being preferred because of its low thermalconductivity, relative inexpensiveness, and general availability. Theterm finely divided silica, as used herein, is intended to apply tonaturally occurring silica as well as the various commercialpreparations of silica, and is not intended to be limited to anyspecific form or preparation of silica.

The radiant heat barrier material 14 of the present invention may beeither a metal, metal oxide, or a metal coated material such as coppercoated mica flakes, or other radiation reflective material, or aradiation opaque material, or a suitable combination of reflective andother absorptive materials, which when mixed with the low conductivepowder, i.e., finely divided silica, will provide a discontinuous seriesof multiple radiation barriers for decreasing and minimizing the passageof heat by radiation through the insulation space. The shape of thebarrier particles should provide a large surface area per unit volume,thin flakes of relatively fine particle size being preferred. Forexample, the radiant heat barrier material may comprise aluminum orcopper in powder or flake form, the latter being preferred.

In addition to the materials already specified as radiant heat barriersin the insulation mixture of the invention, other barriers such ascopper paint pigments, aluminum paint pigments, magnesium oxide, zincoxide, iron oxide, titanium dioxide, copper coated mica flakes, carbonblack and graphite, either alone or in combination with each other, havebeen found to satisfactorily reduce the transmission of infra-redradiation, and to complement the desirably low conductivitycharacteristics of the powder insulation.

It should be understood that while the insulation mixture of the presentinvention comprises two or more component ingredients, it is entirelypossible for the low heat conductive component and the radiation barriercomponent to have the same chemical composition, though widely varyingphysical properties. To illustrate, an insulating powder such as carbonblack in extremely small ultimate particle sizes, i.e., below 0.1micron, gives excellent low heat conductive results, but is deficient asa radiation barrier material. On the other hand, the same carbon blackmaterial in relatively larger particle sizes, i.e., above microns,exhibits greatly improved radiation stopping characteristics, but has ahigh heat conductivity. We have found in general that the substantialelimination of heat flow by conduction through a single componentinsulation by reducing particle sizes may only be accomplished bysacrificing radiation opacity. We have been unable to find any singlecomponent insulation having uniform chemical and physicalcharacteristics and possessing the combined properties of an extremelylow heat conductivity and a high resistance to the transmission ofradiant heat on the order of magnitude achieved by the insulationmixture of the invention.

In the preferred practice of the invention, the choice of particle sizeof the low heat conductive powder should be effective in reducing theheat leak by conduction below the values obtained with prior-knowninsulating powders. Particle sizes may vary up to about 1500 microns.Usually a maximum particle size of 420 microns is desirable foroperation at low boiling liquefied gas temperatures, and for bestoperation particle sizes of 75. microns or smaller may be employed. Thevarious particle sizes enumerated in this specification for the low heatconductive powder component refer to 4 agglomerate particle sizes andnot ultimate particle sizes, unless specifically identified as thelatter.

Tests with insulation mixtures containing radiant heat imperviousparticles having maximum particle sizes of about 500 microns haveproduced very satisfactory results.

Radiant heat barrier material having a particle size below 250 micronshave been tested with excellent results, while particle sizes less than50 microns and flake thickness less than 0.5 micron are preferred.

It is an essential feature of this invention that the insulating powder13 and the radiant heat barrier material 14 to be used in thepowder-vacuum insulation system be thoroughly mixed prior to theirintroduction into the insulating space 11. Only in this fashion is itpossible to maintain a random dispersion of radiant heat barriermaterial throughout the insulation powder, and realize maximum reductionin radiative heat transmission.

While we do not wish to be bound by any particular theory, we believethe reason for the far superior insulating results of the presentinvention resides primarily in the random physical dispersion of radiantheat resisting particles in the insulation mixture. This permits the useof high weight percentages of radiant heat barrier particles to bringabout a marked reduction in radiative heat transmission with only a veryslight increase in heat transfer by conduction.

The striking superiority of the insulation mixture of the presentinvention is believed to be partially attributed to the employment ofsmall particle sizes of low heat conductive insulating powder 13 andradiation barrier flakes 14. This results in an insulation mixture inwhich the barrier flakes 14 are not in close surface contact with theinsulating powder particles 13, but rather are in contact over a reducedsurface area somewhat approaching point contact. The effect of thisrelationship between barrier flakes 14 and low conductive particles 13is to prevent close contact between, and to separate insulatingparticles from each other, and to reduce the tendency for conductiveheat to flow between particles by direct contact over a large contactarea. This also restricts the passage of conductive heat flow across theinsulation space to heat leak paths containing an indefinitely largenumber of exceedingly small contact areas, which offer considerableresistance to the flow of heat therethrough. As a consequence, heatentering the insulation space 11 may be further minimized by anycombination of radiation reflection, radiation absorption by the radiantheat barrier flakes, the relatively high contact resistances betweenlike and unlike particles, as well as the relatively low conductivity ofthe insulating powder.

Thus the mechanism of heat transfer which occurs in a typicalcombination of a particle of finely divided silica and a particle ofaluminum flake might be as follows:

Heat will reach the particles by the modes of radiation and conduction.Of the radiant heat, part will be refiected, part will be absorbed bythe aluminum flake, and the remainder will radiate through and aroundthe particle of finely divided silica. The absorbed radiation will raisethe temperature of the aluminum flake above the temperature of theadjoining finely divided silica par.- ticle. Through the mode of solidconduction, heat will pass from particle to particle and from flake toparticle across the relatively small area of point contact. Thereaftersuch heat will travel by solid conduction across the low conductiveparticle of finely divided silica. By analogy it will be seen that anordinary insulation layer having heat reflecting particles and anindefinitely large number of contact resistances between like and unlikeparticles is particularly efiicient in preventing heat losses byradiation as well as by conduction.

Since the radiant heat barrier particles used in the insulation mixtureof the invention have excellent heat conductive characteristics, itwould seem that an increase in the barrier material contained intheinsulation mixture would impair the conductive insulation efliciency.Contrary to this concept, we have found that substantial increases inthe amount of barrier material continue to be beneficial to the overallinsulating properties of the insulation mixture. In the presentinvention, increasing the amount of aluminum in a mixture of aluminumand silica particles reduces the radiative heat transfer, and onlyslightly increases the heat transfer by conduction. Optimum thermalresistance is obtained when the sum of these two modes of heat transferis at a minimum.

A principal advantage residing in the use of the insulating mixture ofthe present invention is that it is possible to employ a decreasedinsulation thickness without sacrificing the benefits of small exchangeof heat by radiation or conduction. The greater efliciency obtained fromthe use of a smaller insulation thickness arises from the multiplelayers of reflective and absorptive particles available in the particlearrangement of the present invention. In this respect it is to be notedthat the low value of heat transfer rate is only attained when the metalflakes are sufliciently separated by the particles of finely dividedsilica. If the metal flakes contact each other frequently enough, theywill form a solid conductive path.

The thermal transmission from wall to wall of the container is hereinstated in terms of thermal conduc-. tivity K and includes the total heattransfer in B.t.u. per hour, per square foot, per F. per foot.

To indicate still more fully the nature of the present invention, someof the test results of thermal conductivity obtained from insulationmixtures containing aluminum flake and finely divided silica are setforth in Table I. In these tests an insulation space of a double walledcontained was maintained at an absolute air pressure of less than 0.1micron of mercury (0.0001 mm. Hg) and one Wall of the vessel maintainedat a temperature of 183.2 C., while the remaining wall temperature wasadjustably controlled at higher levels. The aluminum flake constituentof the insulation mixture employed in the tests was present in widelyvarying amounts ranging from about 1% to about 80% of the mixture, thepercentage composition being on a weight basis, and based on a mixtureof metal flake having particle sizes between 10 and 50 microns, andinsulating powder particles with agglomerate sizes less than 75 microns.

However, it is to be understood that these tests are presented asillustrative only, and that they are not intended to limit the scope ofthe invention.

Table I h THigher 1%)(10; B.1fi .i1;1./ Material,b wei t errperar., q.

y g ture, C. FJFt.

lminum finel d'vided silica 27.5 3%Au y 1 10.0 0.397

66.5 0. 6% Aluminum-l-finely divided silica 1 9- 0 0.314 40 0.2885 30 10Al minum finel divided silica 20 u y 10 0.246 0 0.232 40 0.211 30 0.21228.6% Aluminum+fine1y divided silica 20 0. 187 10 0,175 0 0.172 40 0.1678&2;

Al m finl divided silica. 20 40% ummu ey 10 0.154 0 0.145 50 0.327 A1 mfinl dividedsilica 30 0. 5 50% uminu ey 20 0.203

Al in m finel dividedsilica 20.0 70% u mu y 1.0 0.464

1 Density of approximately 3 to 6 lbs./cu. it.

As an alternative heat reflective material, copper flakes or copperpowder may be mixed with finely divided silica. Table II below listssome of the results of insulation tests conducted under conditionssimilar to the previouslv described tests using insulation mixturescontaining finely divided silica and copper flakes or powder.

1 Copper in partially oxidized state.

From the results shown in Tables I and II it will be seen that optimumreduction of heat transfer depends on the selection and proportion offinely divided materials used in the insulation mixture. The addition ofsmall percentages of aluminum or cop er flakes to the finely dividedsilica insulation substantially diminishes the thermal heat transferproperties of the mixture. Increasing the percentage of aluminum orcopper admixture further reduces the radiative heat transfer andslightly increases the heat transfer by conduction. An inspection ofTable I will indicate that an optimum mixture is a combination of 40%aluminum flake and 60% finely divided silica, the thermal conductivitybeing 0.163 10' B.t.u./hr., sq. ft., F./ft. Referring to Table II aminimum thermal conductivity occurs at 55.5% copper and 44.5% finelydivided silica, which corresponds to a thermal conductivity of about0.192 10- B.t.u./hr., sq. ft., F./ft. It has also been discovered thatstill lower thermal conductivity figures may be obtained by reducing thecopperoxide surface to copper.

As a feature of the present invention, improved thermal insulation overthe results shown in Table I may be effected by employing barrier flakeshaving particle sizes less than 10 microns. The effect is to shift theoptimum percentage composition of radiant heat barrier material to anumerically higher value, the magnitude of the shift corresponding tothe degree of reduced particle size. For example, by using aluminumflakes sized between 5 and 10 microns, thermal conductivities as low as0.0875 10- B.t.u./hr., sq. ft., F./ft. may be achieved with insulatingmixtures containing 60% aluminum. The same holds true for the heat ratebehavior of similarly sized copper flake-insulation mixtures, thermalconductivities as low as 0.105 10* B.t.u./hr., sq. ft., F./ft. havingbeen obtained with mixtures containing 70% copper by weight.

Thus, depending on particle size, insulation mixtures containing highweight percentages of barrier material,

such as aluminum or copper flakes, may be satisfactorily employed in theinsulation mixture of the invention, even though thealuminum or copperflakes are themselves highly conductive. However, it is to be understoodthat the degree to which the particle sizes of the heat reflecting andabsorbing aluminum bodies are to be reduced is largely controlled by thepractical economics of the situation, particle sizes below 50 micronsbeing preferred.

Table III below is representative of the unexpectedly superior resultsobtained from insulation mixtures of the present invention in comparisonwith the known insulation systems of the prior art.

The unexpected results obtained from the practice of the presentinvention are very impressive. From the above Table III it will be seenthat the passage of heat leak may be minimized by the insulation mixtureof the present invention to as low as .0875 10 B.t.u./hr., sq. ft.,F./ft., which is less than one-tenth the rate of heat transfer obtainedby the most efiicient insulating material in the prior art. Being inpart a function of particle size, still further improvement in thereduction of heat leak may be effected by using even smaller flakesizes.

To further appreciate the advantages of the present invention, it shouldbe noted that in U.S. Patent No. 2,396,459 it was pointed out that forcontainers up to two feet in diameter, vacuum-polished metal surfacesare more efficient and preferable to powder-vacuum insulation. Forlarger sizes, the powder-vacuum insulation is advantageous. As a resultof the significantly low rate of heat transfer possessed by theinsulation mixture of the present invention, insulation layers ofextraordinarily reduced thicknesses may now be advantageously employed,thus reducing the overall dimensions of low temperature storagecontainers for the entire size range of containers, including thoseunder two feet in diameter.

The possibilities in the resultant reduced insulation thickness withinsulation mixtures of the present invention indicate the scope andimportance of this product. For instance, if it is desired to insulate a10 liter spherical container (diameter=l0.5 inches), so that its holdingtime for liquid nitrogen is approximately four weeks (evaporation lossof 3.57% per day), it is to be expected that the necessary insulationthickness using the best known prior art insulating material will beseveral times the diameter, and many times the volume of the uninsulatedcontainer. Theoretical calculations show the required thickness of priorart vacuum-powder insulation having silica powder as the low heatconductive powder to be at least three feet. Expressing this figure interms of volume, the resultant quantity of insulating material would bealmost 500 times the volume of the uninsulated spherical container. Incontrast, an insulation mixture containing 60% aluminum (5 to micronflakes) and the remainder finely divided silica, permits the use of asingularly unusual insulating thickness of less than onehalf inch.

The insulating product of the present invention is even more favorablein situations where longer holding periods may be desired. Thus, if theholding time for the spherical container is increased to 10 weeks, athickness of 1.37 inches of the subject insulating mixture will meetthis rigid specification. However, none of the prior art insulatingmaterials possesses a sufficiently low heat rate to fulfill thisfunction, no matter what thickness of insulating material is used. Thisis because the relatively larger external surface area for heat leakoccasioned by an increase in insulation thickness completely counteractswhatever benefits may be derived from the lengthened heat flow path.

From the foregoing it will be seen that. the thermal heat transfer rateof powder-vacuum insulating material may be materially decreased byuniformly incorporating finely divided infra-red radiation imperviousbodies in a finely divided low heat conductivity powder. The heatimpervious bodies provide a series of heat reflective surfaces forminimizing the transmission of heat radiation through the insulationspace. At the same time the small area contact between like and unlikeparticles providesmaxirnurn thermal resistance to the passage of heat byconduction. Increasing the proportion of radiation retarding bodies,substantially reduces the radiative heat transfer and slightly increasesthe heat transfer by conduction. Through the use of the subject highlyeflicient powder insulation mixture, the required thickness ofinsulation layer may be substantially reduced and the overall containerdimensions minimized.

It will be understood that although the thermal insulation of thepresent invention has been described in connection with powder-in-vacuuminsulating systems for the storage of liquefied gases, the insulation isalso susceptible of use in the preservation of quick frozen biologicalspecimens, living tissues and other perishable commodities, and may beapplied as a thermal insulation at higher temperature levels, at whichconditions the pressure in the insulation. space will not be as criticalor sensitive as at lower temperatures, without departing from the spiritand scope of the novel concepts of the present invention.

What is claimed is:

1. An insulating material characterized by a low rate of heat transferby conduction and radiation, consisting essentially of finely dividedlow heat conductive particles of agglomerate sizes less than about 420microns being selected from the group consisting of silica, perlite,alumina, magnesia and carbon black; and finely divided radiant heatreflecting bodies of sizes less than about 500 microns and havingmetallic surfaces, such radiant heat reflecting bodies constitutingbetween about 1% and by weight of said insulating material.

2. An insulating material characterized by a low rate of heat transferby conduction and radiation, consisting essentially of finely dividedlow heat conductive particles of agglomerate sizes less than about 420microns being selected from the group consisting of silica, perlite,alumina, magnesia and carbon black; and finely divided radiant heatreflecting bodies of sizes less than about 500 microns and constitutingbetween about 1% and 80% by weight of said insulating material, saidheat reflecting bodies consisting of at least one member selected fromthe group consisting of aluminum, copper, aluminum paint pigments,copper paint pigments and copper coated mica.

3. An insulating material characterized by a low rate of heat transferby conduction and radiation, consisting essentially of finely dividedlow heat conductive particles. of less than about 420 micronsagglomerate size being selected from the group consisting of silica,perlite, alumina, magnesia and carbon black; and finely divided radiantheat reflecting bodies of less than about 500 microns size, said radiantheat reflecting bodies consisting of aluminum flakes in an amountbetween about 1% and 80% by weight of said insulating material.

4. An insulating material characterized by a low rate of heat transferby conduction and radiation, consisting essentially of finely dividedlow heat conductive particles of less than about 420 microns agglomeratesize being selected from the group consisting of silica, perlite,alumina, magnesia and carbon black; and finely divided radiant heatreflecting bodies of less than about 500 microns size, said radiant heatreflecting bodies consisting of copper flakes in an amount between about1% and 80% by Weight of said insulating material.

5. An insulating material characterized by a low rate of heat transferby conduction and radiation, consisting essentially of finely' dividedsilica particles of less than about 75 microns agglomerate size; andfinely divided radiant heat reflecting bodies of less than about 250microns size and constituting between about 1% and 80% by weight of saidinsulating material being uniformly interspersed in said low conductiveparticles, said heat reflecting bodies consisting of at least one memberselected from the group consisting of aluminum, copper, aluminum paintpigments, copper paint pigments and copper coated mica.

6. An insulating material characterized by a low rate of heat transferby conduction and radiation, consisting essentially of finely dividedsilica particles of less than 75 microns agglomerate size; and finelydivided radiant heat reflecting aluminum flakes of less than about 50microns size in an amount between about 1 and 80% by weight of saidmaterial.

7. An insulating material characterized by a low rate of heat transferby conduction and radiation, consisting essentially of finely dividedsilica particles of less than about 75 microns agglomerate size; andfinely divided radiant heat reflecting copper flakes of less than about50 microns size in an amount between about 1 and 80% by weight of saidinsulation material.

8. In combination with a vacuum insulating system, a mixture of finelydivided low heat conductive particles so reduced in agglomerate size toless than about 420 microns as to substantially impede heat inleak byconduction and yield to the predominant passage therethrough of heatinleak by radiation, said low conductive particles being seelcted fromthe group consisting of silica, perlite, alumina, magnesia and carbonblack; and finely divided radiant heat reflecting bodies of less thanabout 500 microns size and having metallic surfaces, such radiant heatreflecting bodies constituting between about 1% and 80% by weight ofsaid mixture, whereby said system affords a high resistance to heatinleak by all modes of heat transfer.

9. In combination with a vacuum insulating system, a mixture of finelydivided silica particles of less than about 75 microns agglomerate size;and finely divided aluminum flakes of less than about 50 microns sizeand having thicknesses of less than 0.5 micron, said aluminum flakesbeing 10 present in an amount between about 1 and 80% by weight of saidmixture.

10. In combination with a vacuum insulating system, a mixture of finelydivided silica particles of less than about 75 microns agglomerate size;and finely divided copper flakes of less than about microns size andhaving thicknesses of less than 0.5 microns, said copper flakes beingpresent in an amount between about 1% and 80% by weight of said mixture.

11. An insulating material characterized by a low rate of heat transferby conduction and radiation, consisting essentially of finely dividedlow heat conductive particles of agglomerate sizes less than aboutmicrons and ultimate particle sizes less than about 0.1 micron beingselected from the group consisting of silica, perlite, alumina, magnesiaand carbon black; and finely divided radiant heat reflecting metalflakes of sizes less than about 50 microns, such radiant heat reflectingflakes constituting between about 1% and by weight of said insulatingmaterial.

12. An insulating material characterized by a low rate of heat transferby conduction and radiation, consisting essentially of finely dividedsilica particles of less than about 75 microns agglomerate size andhaving a density of about 3 to 6 lbs. per cu. ft.; and finely dividedradiant heat reflecting flakes of sizes less than about 50 microns andhaving metallic surfaces, such radiant heat reflecting flakesconstituting between about 1% and 80% by weight of said insulatingmaterial.

References Cited in the file of this patent UNITED STATES PATENTS1,764,311 Hunt June 17, 1930 2,093,454 Kistler Sept. 21, 1937 2,313,379Wood Mar. 9, 1943 2,459,282 McDougal Jan. 18, 1949 2,553,016 Sosnick May15, 1951 Disclaimer 2,967,l52.-Ladslas C. M atsoh, Kenmore, and Amhm" W.Francis, Dobbs Ferry, -N.Y. THERMAL INSULATION. Patent dated J an 3,1961. Disclaimer filed Aug. 30, 1963, by the assignee, Union Uaw'bideOwpomtz'oh. Hereby enters this disclaimer to claims 1, '2, 3, 4 and 8 ofsaid patent.

[Ojficial Gazette October 22,1963]

2. AN INSULATING MATERIAL CHARACTERIZED BY A LOW RATE OF HEAT TRANFER BYCONDUCTION AND RADIATION, CONSISTING ESSEENTIALLY OF FINELY DIVIDED LOWHEAT CONDUCTIVE PARTICLES OF AGGLOMERATE SIZES LESS THAN ABOUT 420MICRONS BEING SELECTED FROM THE GROUP CONSISTING OF SILICA, PERLITE,ALUMINA, MAGNESIA AND CARBON BLACK; AND FINELY DIVIDED RADIANT HEATREFLECTING BODIES OF SIZES LESS THAN ABOUT 500 MICRONS AND CONSTITUTINGBETWEEN ABOUT 1% AND 80% BY WEIGHT OF SAID INSULATING MATERIAL, SAIDHEAT REFLECTING BODIES CONSITING OF AT LEAST ONE MEMBER SELECTED FROMTHE GROUP CONSISTING OF ALUMINUM, COPPER, ALUMINUM PAINT PIGMENTS,COPPER PAINT PIGMENTS AND COPPER COATED MICA.